Amino acid metabolism IV. Biosynthesis of nonessential amino acids Figures: Lehninger-4ed; fejezet: 22 (Stryer-5ed; fejezet: 24)

Overview of nitrogen metabolism Nitrogen-fixing nodules: (a)root nodules of a legume (b)thin section through a pea root nodule The nitrogen cycle

Nitrogen fixation: N 2 + 3H 2 → 2NH 3 (exergonic:  G’ o = -33,5kJ/mol) The N  N triple bond is very stable  the activation energy of the N-fixation is extremely high! In biological N-fixation this is accomplished by the binding (to the nitrogenase enzyme complex) and hydrolysis of 16 ATP molecules. N H + + 8e¯ + 16ATP → 2NH ADP + 16Pi + H 2

Incorporation of ammonia into biomolecules through glutamate and glutamine: (1)Glu + ATP →  -glutamyl phosphate + ADP (2)  -glutamyl phosphate + NH 4 + → Gln + Pi + H + (1)+ (2) Glu + NH ATP → Gln + ADP + Pi + H + (glutamine synthetase reaction: biosynthesis of glutamine) Glutamine synthetase: primary regulation point 12 identical subunits regulated allosterically and by covalent modification (covalent modification: adenylation by adenylyltransferase  enzyme inactivation)

Allosteric regulation of glutamine synthetase: each inhibitor alone  produces partial inhibition all 8 inhibitors together  shut down the enzyme

Biosynthesis of glutamate: 1.) transamination of  -ketoglutarate (amino acid degradation) 2.) minor pathway (the reaction equilibrium favors reactants):  -ketoglutarate + NH NAD(P)H ↔ L -Glu + NAD(P) + + H 2 O ( L -glutamate dehydrogenase reaction: liver, mitochondr. matrix)

Major classes of reactions in the biosynthesis of amino acids and nucleotides: 1.) transamination reactions (PLP) 2.) one-carbon group transfer reactions (THF, adoMet) 3.) transfer of amino groups derived from the amide nitrogen of glutamine (glutamine amidotransferase)

Overview of amino acid biosynthesis Nonessential amino acids: Ala, Asp, Asn, Glu, Ser Conditionally essential amino acids: Pro, Arg, Gly, Cys, Gln, Tyr Nitrogen sources: Glu, Gln

5-phosphoribosyl-1-pyrophosphate (PRPP): notable intermediate in the biosynthetic pathway of His and Trp, and nucleotide synthesis Ribose-5-phosphate + ATP → 5-phosphoribosyl-1-pyrophosphate + AMP (ribose phosphate pyrophosphokinase reaction)

Biosynthetic pathways of nonessential and conditionally essential amino acids in mammals

Biosynthesis of proline (also in bacteria)

Another biosynthetic pathway for proline in mammals: dietary or tissue protein → arginine arginine → ornithine + urea (urea cycle: arginase) In the case of insufficient Arg levels the enzyme operates in the direction of ornithine formation! (ornithine → citrulline →→ arginine, urea cycle! )

The major pathway for the formation of Ser is the same in all organisms! Ser is the precursor of Gly and Cys!

Biosynthesis of serine and glycine

In the liver of vertebrates Gly can be synthetised by another route: CO 2 + NH N 5,N 10 -methylenetetrahydrofolate + NADH + H + ↔ glycine + tetrahydrofolate + NAD + (glycine synthase or glycine cleavage enzyme)

Biosynthesis of cysteine (Met → S-atom, Ser → C-skeleton)

Biosynthesis of alanine, aspartate and asparagine Aminotransferase reactions (PLP): Glu + pyruvate ↔ Ala +  -ketoglutarate Glu + oxaloacetate ↔ Asp +  -ketoglutarate Asp + Gln → Asn + Glu (glutamine amidotransferase) The Met, Thr, Lys, Ile, Val and Leu are essential amino acids, their biosynthetic pathways (bacteria!) are complex and interconnected!

Biosynthesis of tyrosine

The amino acid biosynthesis is under allosteric regulation!  The regulatory enzyme is usually the first in the sequence and it is allosterically inhibited by the end product of the pathway! („feedback inhibition”)